The present invention relates to an inking type wire dot printer which dips the tips of printing wires in ink to perform printing of dot characters, and more specifically to an ink supply device which can control a quantity of ink supplied to the printing wires to supply an optimal quantity of ink to the printing wires.
The inking type wire dot printer is designed so that an ink occlusion body as an ink dipping portion composed of felt or sponge is provided at the extreme end of a printing head, printing wires are inserted through the ink occlusion body to dip the tips of the printing wires in ink, and dot-like characters are printed on a sheet of recording paper with the ink. In this inking type dot printer, the quantity of ink within the ink occlusion body decreases as the printing progresses, and therefore, ink is not always sufficiently applied to the wires unless some measures are taken, thus deteriorating the printing quality. To overcome this, in the past, a device is provided to automatically supply ink to the printing head to always maintain the quantity of ink within the ink occlusion body constant so as to always keep the quality of printing at an optimal condition.
In prior art ink supply devices, a pair of detection electrodes are provided in a spaced relation within the ink occlusion body, whereby an AC voltage is applied to one electrode, and the other electrode utilizes the ink in the ink occlusion body as a transfer medium of voltage to detect the voltage. Since this voltage varies with the quantity of ink, a supply of ink from an ink tank is stopped when the detection voltage reaches a predetermined upper value. Therefore, for example, it is controlled so that when the magnet valve is closed to assume a required lower limit value, the magnet valve is opened so as to supply ink from the ink tank to the ink occlusion body. However, the other electrode has a detection voltage which is 0 volt unless the ink occlusion body contains ink to some extent, and thereafter the detection voltage abruptly rises. However, this rise in voltage merely indicates that the pair of electrodes have assumed the conductive condition but the quantity of ink required for printing has not been supplied to the ink occlusion body. When ink is thereafter kept supplying, the detection voltage slowly rises and then the detection voltage is maintained at a certain level. At this time, a quantity of ink enough to provide optimal printing is supplied to the ink occlusion body, and optimal printing is not performed until the upper limit value and lower limit value are set from a voltage value in the vicinity of the maintained voltage. However, in this case, there is a little differential in voltage between the upper limit value and lower limit value, and variation in detection voltage resulting from change in temperature relatively increases resulting in a possible error in detection.
In an attempt of preventing the occurrence of detection error of the quantity of ink resulting from a change in temperature, a proposal has been made wherein a comparing ink occlusion body which is formed of a material similar to that of the ink occlusion body provided on the printing head is provided within an ink tank, the comparing ink occlusion body also having detection electrodes provided thereon so that a difference between the detection voltages of both ink occlusion bodies is obtained to effect temperature compensation, and the quantity of ink is sensed from the voltage differential to control a supply of ink. However, the ink tank is normally loaded on a carriage and as printing progresses, the ink tank is also moved. The ink tank has a space above a liquid level therein, and when the ink tank is moved, the ink rolls to disturb the flow of ions between the detection electrodes of the comparing ink occlusion body, which leads to variation in detection voltage for temperature compensation, thus bringing forth erroneous operation of supplying ink. Moreover, the quantity of ink within the ink tank is much greater than the quantity of ink of the ink occlusion body of the printing head, and therefore, heat transfer on the side of the ink tank is deteriorated and the change in temperature becomes different from that of the ink occlusion body of the printing head. Because of this, there occurs a difference in the temperature characteristic between both the ink occlusion bodies, resulting in a possible error in detection.